Video imaging systems are used in a broad variety of applications, including telecommunications, entertainment, security monitoring, and the like. Video imaging systems are generally placed in one of two categories, namely computer and television. In television applications, baseband video signals can follow a number of different formats, including the National Television Systems Committee or NTSC standard (the U.S. and Japan standard), the Phase Alternating Line or PAL standard (the European standard), and the High Definition/Standard Definition Digital Television format.
In television, there are three basic levels of baseband signal interfaces. In order of increasing quality, they are composite video (or Color, Video, Blanking, and Sync or CVBS), which uses one wire pair, Y/C (or S-video), which uses two wire pairs, and component, which uses three wire pairs. Each wire pair includes a signal and a ground. Composite signals are the most commonly used analog video interface and combine the brightness information (luma) and the color information (chroma) and the synchronizing signals on just one cable.
FIG. 2 depicts the horizontal blanking portion of a typical NTSC composite video waveform that includes color information and represents one horizontal scan line. The signal includes a front porch 200 (which is the area of the signal between the end of the active video and the leading edge of a sync pulse), the sync pulse or tip 204, (which provides synchronizing timing information), a breezeway 208 (which is the area of the signal representing the time between the rising edge of the synch pulse 204 and the start of a color burst 212, the color burst 212 (or color subcarrier) (which is 8-10 cycles of the color reference frequency and is positioned between the rising edge of the sync pulse and the start of the active video), and the back porch 216 (which is the time between the end of the color burst and the start of active video).
In video imaging and video recording systems, analog composite video signals are often converted to a digital video signal for subsequent enhancement, display, and/or recording. While it is desirable that the analog composite video signal input to an analog-to-digital (A/D) converter fully meets the appropriate composite video standard (either the NTSC or PAL standards), in practice the composite video signal is often degraded and does not meet the desired standard due to some combination of low signal output from the video camera, signal loss, interference, or improper AC or DC signal levels. When the analog composite video signal fed to the A/D converter is degraded, the resulting digital video signal may be distorted. For example, the sync tip 204 may be disfigured. This distortion may be very slight or severe, depending on the type and level of degradation to the analog video signal.
The most common causes of composite video signal distortion in video imaging and video recording systems are as follows:                Signal loss in the transmission means. This loss may be due to attenuation, mismatched transmission lines, splitters or other passive devices within the transmission path.        
Low output from the video source. If the signal output from the video camera or other source is not of sufficient amplitude, the video signal at the input to the A/D converter may not meet the desired standard even if the transmission line is virtually lossless.
Interference due to ground loops. Ground loops result when the ground potential is different between two components within a video system. This difference in ground potential can result in “hum bars” on the video picture caused by 60 Hz commercial power, or herringbone interference on the video picture caused by AM broadcast signals, or a combination of 60 Hz and AM broadcast interference that can result in hum bars, herringbone, blanking, color distortion, or other degradation to the video picture.                DC Offset. A DC voltage may be superimposed on the video signal by the video source or by another device within the signal path. If sufficiently large, the DC offset can result in physical damage to the A/D conversion means and/or to other components within the system. Even a small DC offset may not cause noticeable degradation to the video picture in an analog system but may cause errors in the A/D conversion process as described above. Many video components utilize AC coupling to eliminate DC offset, however, this approach results in the signal not having a fixed (e.g., DC) reference, which is critical to producing an analog composite video signal that fully meets the NTSC or PAL specifications.        
One exemplary application for the present invention is video security systems, such as that shown in FIG. 1. Such systems typically incorporate a multiplicity of video cameras 100 in continuous operation. The analog composite video signals from these cameras are transmitted via a transmission line 104 (e.g., coaxial cable, twisted pair, fiber optics, or radio being the most common transmission line) to a central location where the video signals may be monitored and are recorded. The video monitors and recorders 108 at the central location most commonly accept baseband composite video signals following either the NTSC format or the PAL format. Until recently, most video security systems utilized analog recorders. New systems and upgrades to existing video security systems typically incorporate one or more digital video recorders, or DVRs, that are designed to accept analog baseband video signals. A/D conversion circuitry 112, which is typically integrated within the DVR, is designed to work with either or both of the two signal standards (NTSC and PAL).
When the input video signal is even slightly degraded and does not meet the desired standard, the A/D conversion process may distort the digitized video signal, causing the resultant video picture to be significantly worse in terms of picture quality than would be the case with an analog video recorder. Whereas an analog video recorder might record a useable, though degraded, picture, the picture recorded by the DVR may be distorted to such an extent that much or all of the information is lost. Additionally many DVRs are designed to detect motion within the video picture based on an algorithm performed on the digitized video signal. If the digitized video signal is not an accurate and true representation of the video picture captured by the video camera, then the DVR algorithm may falsely detect motion when no motion is present, or conversely not detect motion when motion is actually present. In the first instance, the DVR will rapidly fill its available digital storage medium resulting in loss of data due to either insufficient storage capacity or earlier data being overwritten; in the second instance, the DVR will not record the desired data at all.
The common causes of video signal degradation within a video security system are well understood by video engineers and other experienced technical people. However, video security system installers typically have neither the necessary training to know the causes of video signal degradation, the needed test equipment (or training) to identify the specific cause of degradation in a video system installation, nor the knowledge to determine the most cost-effective solution even if the cause of a problem is identified. Additionally, in many installations the level of video signal degradation would be acceptable with an analog video recorder but results in severe picture degradation, continuous recording, or no recording, when a DVR is installed. There is currently no system able to simultaneously correct many of the common causes of video signal degradation within a video security system. With the increasing use of digital video recorders in video security systems, the need for such a solution is immediate and growing.
Another application for the present invention is video imaging systems. Analog composite video cameras are commonly used to capture video pictures that are converted to a digital video signal, then digitally manipulated or enhanced, displayed and/or recorded. Such applications, which include medical, satellite and airborne imaging systems, utilize an A/D conversion process as described above, and these systems may suffer from the same distortion effects described above if the analog video signal input is degraded. Since imaging systems typically require the highest possible picture resolution, any loss of intelligence due to degradation of the analog composite video system is highly undesirable.
Another application for the present invention is radar system displays. In many radar systems, the output from the radar receiver is a composite video signal. Many radar systems digitally enhance the displayed signal. Such systems utilize an A/D conversion process as described above, and these systems may suffer from the same distortion effects described above if the analog video signal input is degraded. Such display systems are utilized to produce the highest possible picture resolution, and the loss of intelligence due to degradation of the analog composite video system is highly undesirable.